An ion beam by means of a liquid metal is expected to be very useful, since it makes fine work to a level of nanometers possible, while the fine work by conventional electron beam exposure has a limit at a level of submicrons. Further, the ion beam is applicable to mask-less ion implantation, milling or X-ray mask repairs.
On the other hand, a field emission electron beam or a thermal field emission electron beam is expected to be useful as an electron source for a low acceleration SEM used for e.g. observation of minute portions of semiconductor devices.
Heretofore, various structures have been proposed for liquid metal ion sources.
For instance, (1) an electric field evaporation type ion source structure has been proposed wherein a hair pin type heater is used in which a needle-like electrode is secured to a bent portion of the heater (Japanese Unexamined Patent Publication No. 114257/1981). However, in this structure, the hair pin type heater undergoes a thermal deformation, when heated, whereby the position of the needle-like electrode changes, and a stable ion beam is hardly obtainable.
As an improvement, (2) a structure has been proposed in which a reservoir for a metal to be ionized is supported by a conductive support member. More specifically, this liquid metal ion source structure comprises a reservoir for a metal to be ionized, a needle-like electrode having a needle-like tip to which a liquid metal is supplied from the reservoir, a means for applying a strong electric field to the needle-like tip, and a heat-generating support member thermally connected to the reservoir or to the needle-like electrode and being capable of generating a heat when electrically energized, the supporting member being formed by a material having a poor affinity with the liquid metal (Japanese Unexamined Patent Publication No. 35829/1983).
Further, (3) a structure wherein, an electrode extends through the center of a plate-like heat-generating member, and the above-mentioned material is attached around the extending-through portion (Japanese Unexamined Patent Publication No. 101750/1984).
However, these methods have a drawback that even when the heat-generating member is made of a material hardly wettable with the substance to be ionized, such a substance tends to overflow from the reservoir or from the needle-like electrode as ions are continuously emitted by conducting an electric current, and the surface of the heat-generating member will be wetted by the substance, whereby the electric resistance of the member tends to be low, and it eventually becomes difficult to conduct heating, thus adversely affecting the quantity of the ion beam emission.
Further, the needle-like electrode is made of a material having a high melting point and being usually brittle, and it was difficult to bond the reservoir or the heater by a conventional simple method such as spot welding. As shown in Japanese Unexamined Patent Publication No. 132632/1982 (corresponding to U.S. Pat. No. 4,467,240), in a structure wherein a needle-like electrode is secured to a carbon heater, no practical reservoir exists, and an evaporation loss of the substance to be ionized is substantial, whereby the life of the ion source is short.
On the other hand, in the conventional field emission electron source or thermal field emission electron source, it is common that the tip of single crystal tungsten is shaped into a needle-like shape by electropolishing, and an electric field is applied to the tip to obtain electron emission. The field emission electron source is required to be under a highly vacuumed condition at a level of not higher than 10.sup.-10 torr, and a high level of technique is required to maintain the highly vacuumed condition. However, conventional electron source structures have a drawback that it is impossible to maintain an electron beam in a stabilized condition unless a flushing operation by heating the needle-like electrode at a high temperature is conducted from time to time.
For a thermal field emission electron source, it is known that a needle-like electrode tip made of a tungsten single crystal having a &lt;100&gt; orientation and covered with metal zirconium, followed by heat treatment in the presence of a very small amount of oxygen to have the work function of the (100) plane reduced from 4.5 eV to 2.5 eV (U.S. Pat. No. 3,814,975). However, this method has a drawback that a complicated operation is required for the activating treatment to reduce the work function of the tip.